Polymeric Reagents and Catalysts - ACS Publications - American

5 Stability of Polymer-Supported Transition Metal Catalysts

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/bk-1986-0308.ch005

Philip E. Garrou Central Research-New England Laboratory, Dow Chemical USA, Wayland, M A 01778

During the last 15 years chemists have been enamoured with the idea of anchoring transition metal catalysts to organic polymers. Such studies have sought to produce heterogenized catalyst systems that are as active and selective as their homogeneous counterparts while having the distinguishing characteristic of being easily separable from the reaction media. As polymer supported catalysts are virtually certain to be more expensive than their homogeneous analogues, it is vital that they be recycled. Most researchers have been aware of the need to recycle but unfortunately very few studies have determined an activity vs. time relationship for such immobilized catalyst systems. It is difficult to accurately estimate the operating cost (cost of catalyst per unit of product) of a given polymer immobilized catalyst system. Such costs depend not only on the primary cost of the transition metal species and the functionalized support, but also on the catalysts activity and the length of its working life. Such factors can only be determined by experimentation for each individual case. While the cost/unit metal is about the same for polymer immobilized homogeneous catalysis and conventional heterogeneous catalysts, the cost of the matrix w i l l obviously be higher for the immobilized systems (1_). One must therefore show some unique activity or selectivity vs. conventional heterogeneous catalysts in order to warrant use of the polymer immobilized systems. It must be remembered, however, that use of such materials in industrial chemical processing necessitates knowledge of the long term stability of such catalyst systems since reactor downtime to unload, regenerate or replace the catalyst can dramatically impact the "operating cost" of the catalyst system. While typical publications i.i the field describe systems that "lose l i t t l e activity upon recycling" one look at the data presented in Figure 1 reveals that even minimal activity losses of 3% per day could severely limit the useful lifetime of immobilized catalyst systems (2). Another such evaluation is depicted in Figure 2. This plot depicts the relative price for sale of a typical hydroformylation product vs the projected catalyst lifetime at a constant loss of 1 ppm Rh catalyst in the effluent stream (3). It is clear that 60+ days (1,440 hours) of continuous usage would be needed to approach the full economic potential of this supported catalyst system. 0097-6156/86/ 0308-O084S06.75 / 0 © 1986 American Chemical Society

In Polymeric Reagents and Catalysts; Ford, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


5.

GARROU

Stability

of Polymer-Supported

100 LIFETIME

Transition Metal

200

Catalysts

85

300

(DAYS)

Figure 1. A c t i v i t y vs. l i f e t i m e of immobilized catalyst systems. Reproduced with permission from reference 2. Copyright 1980 Springer-Verlag Heidelberg.

Figure 2. Relative price for sale of a t y p i c a l hydroformylation product vs. projected catalyst lifetime at a constant loss of 1 ppm Rh catalyst i n the effluent stream.



86

P O L Y M E R I C R E A G E N T S A N D CATALYSTS

The b a s i c q u e s t i o n that has to be answered f o r each i n d i v i d u a l reaction examined i s whether or not the products produced are of high enough value to warrant the use of i m m o b i l i z e d c a t a l y s t s . I t i s l i k e l y that the commercial impact of such catalyst systems w i l l be greatest i n the pharmaceutical area where the s e l e c t i v i t i e s inherent i n homogeneous c a t a l y s i s are needed, the v a l u e of the products i s high and the a b i l i t y to e a s i l y separate the c a t a l y s t could pay f o r i t s e l f after a l i m i t e d number of recycles. Impact i n t h i s area has not been obvious, probably because the polymer immobilized catalyst systems have not been i n the hands of the t y p i c a l synthetic organic chemist • As we have mentioned, a n a l y s i s of the modes by which c a t a l y s t d e a c t i v a t i o n occurs i s important to understanding where such c a t a l y s i s c o u l d b e s t be a p p l i e d . I t has become c l e a r t h a t d e a c t i v a t i o n can occur by any or a l l of the f o l l o w i n g mechanisms: (a) " l e a c h i n g " of the c a t a l y s t from the support due to the l a b i l i t y of the t r a n s i t i o n metal-functionality bond; (b) chemical i n s t a b i l i t y of the support (both backbone and f u n c t i o n a l i t y ) under r e a c t i o n conditions; (c) p r o d u c t i o n of metal c r y s t a l l i t e s i n the polymer matrix under reductive conditions. Lability Metal loss may occur i f the ligands by which the complexes are bound to the polymer undergo reversible d i s s o c i a t i o n during the reaction. For example, the w e l l known Wilkinson catalyst, RhCKPPh^)^, i s known to d i s s o c i a t e a t e r t i a r y phosphine l i g a n d i n order to become c a t a l y t i c a l l y a c t i v e . I f the phosphine bound c a t a l y s t d e p i c t e d i n Equation 1 underwent such O

-PPh RhCl(PPh ) 2

3

2

-

* Q

@)-PPh

2

+ RhCl(PPh ) 3

( 1 ) 2

dissociation, loss of Rh to the solution would obviously r e s u l t . If such catalysts were used i n a continuous flow reactor, rhodium would be slowly drained from the catalyst system. Increasing the ligand to metal r a t i o w i l l i n h i b i t d i s s o c i a t i o n r e a c t i o n s but w i l l a l s o obviously exhibit an i n h i b i t i n g effect on the rate of the reaction. Chemical I n s t a b i l i t y It has generally been assumed that the bonds that l i n k the catalyst to the polymer support are c h e m i c a l l y s t a b l e under the r e a c t i o n c o n d i t i o n s one employs. U n t i l r e c e n t l y , the l i t e r a t u r e o f f e r e d l i t t l e information i n this regard, since l i f e t i m e studies are needed to p r o p e r l y evaluate s t a b i l i t y . Recent publications have pointed out the c h e m i c a l i n s t a b i l i t y of the phosphorus-carbon bond of t e r t i a r y p h o s p h i n e f u n c t i o n a l i z e d supports and the chemical r e a c t i v i t y of v a r i o u s n i t r o g e n f u n c t i o n a l i z e d p o l y m e r i c support m a t e r i a l s under r e a c t i o n c o n d i t i o n s . I f such chemical s t a b i l i t y problems are present, the consequences are indeed s e r i o u s . While a t y p i c a l "leach" s i t u a t i o n would necessitate a periodic reloading of the metal complex, c l e a v a g e of p o l y m e r f u n c t i o n a l i t y w o u l d necessitate replacement of both the metal complex and the polymer.


5.

GARROU

Stability


Transition

Metal

Catalysts

87


Metal C r y s t a l l i t e Formation C l e a r l y the most o b s e r v e d p r o b l e m of p o l y m e r i m m o b i l i z e d hydrogenation c a t a l y s t s i s the decomposition of the anchored o r g a n o m e t a l l i c complexes to metal p a r t i c l e s . I n t e r p r e t a t i o n of l i t e r a t u r e o b s e r v a t i o n s are complex s i n c e c a t a l y s t s a r e not d e a c t i v a t e d , but r a t h e r r e v e a l c a t a l y t i c a c t i v i t y u s u a l l y only observed for heterogeneous catalysts. C r y s t a l l i t e s can sometimes be d e t e c t e d by v i s u a l i n s p e c t i o n f o r darkening. However, e l e c t r o n microscopy i s a more r e l i a b l e d e t e c t i o n method. The presence of small amounts of metal i n homogeneous catalyst systems has recently been reviewed by Laine (4). The presence of active " i n v i s i b l e " metal c o l l o i d s i n homogeneous catalyst systems has been the topic of much recent a c t i v i t y by Crabtree and co-workers (£). In general, reports that observe the presence of an i n d u c t i o n p e r i o d and/or unusual c a t a l y t i c a c t i v i t y such as arene hydrogenation, conversion of syn-gas to hydrocarbons or n i t r i l e reduction to amines, should be treated as suspect since such r e s u l t s are usually indicative of c o l l o i d or metal c r y s t a l l i t e formation. In general i t appears that the polymer matrix enhances t h i s decomposition problem. Reports of c r y s t a l l i t e f o r m a t i o n , under r e a c t i o n c o n d i t i o n s where the corresponding homogeneous systems are known to be stable, continue to appear i n the l i t e r a t u r e . I t has been shown that such c a t a l y s t decomposition i s retarded at high P/M r a t i o s , but again this usually retards the rate of the reaction. What follows i s a review of immobilized catalyst research that has s p e c i f i c a l l y addressed the question of catalyst s t a b i l i t y . The l i t e r a t u r e coverage i s s e l e c t i v e , but comprehensive enough to present an accurate picture on the current state of research i n t h i s area. Phosphorus

Functionality

Haag and Whitehurst (6.7) f i r s t described the use of weak base anion exchange resins containing phosphorus and nitrogen f u n c t i o n a l i t y as supports for organometallic complexes i n the early 70*8. Because of i t s dominant r o l e i n numerous c a t a l y t i c r e a c t i o n s (8.9) t r i v a l e n t phosphorus has been the the predominant l i n k by which metal complexes have been a t t a c h e d t o a v a r i e t y of o r g a n i c p o l y m e r s ( 1 0 ) . Crosslinked phosphinated polystyrene has been the predominant support due both to i t s commercial a v a i l a b i l i t y and to the ease with which i t can be prepared i n the academic l a b o r a t o r y (11). Due to ease of p r e p a r a t i o n and b e t t e r a i r s t a b i l i t y , 1 and 2 have been the l i g a n d s most frequently studied although 3 and more recently 4 are available (12.13). L a b i l i t y of the Metal-Phosphorus Bond By f a r the most complete study on e l u t i o n or l e a c h i n g of metal complexes under " i n d u s t r i a l " flow c o n d i t i o n s was r e p o r t e d by the



88

P O L Y M E R I C REAGENTS A N D CATALYSTS

Mobil group (14) who examined o l e f i n hydroformylation catalyzed by Rh complexes supported on P-C^PU-Bu)?. The catalyst resins contained 2-3 meq/g phosphorus and 0.2 meq/g Rh. C a t a l y t i c r e a c t i o n s were carried out i n a tubular down-flow reactor. They observed that the rhodium c o n c e n t r a t i o n s i n s o l u t i o n over the phosphine r e s i n s were p r o p o r t i o n a l to the percent l o a d i n g of the metal, i n d i c a t i n g an e q u i l i b r i u m was e s t a b l i s h e d . As s o l v e n t , s u b s t r a t e or product p o l a r i t y increased the rhodium concentration i n s o l u t i o n i n c r e a s e d . A h i g h e r c o n c e n t r a t i o n of rhodium was a l s o noted when there was a higher CO p a r t i a l pressure and/or the temperature was decreased as shown i n Table 1. A most important glimpse of the dynamics of a working catalyst was obtained by examining the catalyst concentration i n the reactor bed as a f u n c t i o n of time. F i g u r e 3 r e v e a l s a d e p l e t i o n of Rh w i t h time as a f u n c t i o n of the d i s t a n c e down the bed. I t i s c l e a r that d e p l e t i o n o c c u r s i n t h e d i r e c t i o n of f l o w . For propylene hydroformylation under the given reaction conditions, 48% of the Rh was l o s t i n 30 days. I t was assumed that a l l of the rhodium l o s s was due to d i s s o c i a t i o n of the rhodium-phosphorus bond. These systems were not examined f o r cleavage of the phosphorus from the r e s i n , nor was there any mention of special attempts to exclude the last traces of 0 from the catalyst beds, both of which, as we s h a l l discuss below, are potential problems. B r i t i s h Petroleum group (15.16) also examined the use of rhodium complexes bonded to 1 and 5 f o r the hydroformylation of hexene-1. Reactions were c a r r i e d out i n e i t h e r s t i r r e d a u t o c l a v e s or a single pass t r i c k l e flow reactor at 80-90 °C and 650 psig of C0/H They noted that s t r i c t e x c l u s i o n of 0 from the r e a c t i o n mixture lowered the Rh c o n c e n t r a t i o n i n the e f f l u e n t from 8 to 2 ppm i n autoclave studies. A continuous flow p i l o t plant test of a catalyst c o n s i s t i n g of 2.2% Rh loaded onto 1 r e v e a l e d up to 90 ppm Rh i n the products a f t e r 24 hours on stream. When such experiments were repeated under r i g o r o u s l y oxygen-free conditions the Rh content of the p r o d u c t s fell to 1 ppm. Higher c o n c e n t r a t i o n s of oxygenodes/hydrocarbons 1:2 v/v, resulted i n 7 ppm Rh i n the l i q u i d eluate. Rhodium c a t a l y s t s supported on 5 were i n s o l u b l e i n hydrocarbons but exhibited s i g n i f i c a n t s o l u b i l i t y i n aldehydes which f a c i l i t a t e d the loss of rhodium from the reactor. It was concluded that even ppm l e v e l s of 0 i n the feedstocks would g i v e r i s e to Rh e l u t i o n due to l o s s of f u n c t i o n a l i t y because of rhodium c a t a l y z e d t e r t i a r y phosphine oxidation to t e r t i a r y phosphine oxide. 2

2#

2

2

Chemical Reactivity of the Metal-Phosphorus Bond As noted above one source of chemical r e a c t i v i t y i s the observation of phosphine o x i d a t i o n which lowers the a v a i l a b i l i t y of l i g a t i n g f u n c t i o n a l i t y . Another mode of deactivation i s actual rupture of the phosphorus-carbon bond. In order to understand the c h e m i c a l i n s t a b i l i t y of t r a n s i t i o n metal phosphine complexes on r e s i n s , we must f i r s t e x a m i n e some r e c e n t d a t a on t h e c h e m i c a l s t a b i l i t y of t e r t i a r y phosphines d u r i n g homogeneously c a t a l y z e d



5.

GARROU

Stability

T a b l e 1.

Feed

Benzene

a 1-Hexene b 2-Ethylhexanal

a)


Transition

Metal Catalysts

89

E f f e c t o f V a r i a b l e s on Rh C o n c e n t r a t i o n i n S o l u t i o n

H (psig)

CO(spig)

Temp(°C)

1,000

1,000

100

0.08

1.5

1,000

1,000

100

0.64

8.0

750

750

85

0.06

4.0

750

750

85

1.02

13.0

1,000

1,000

100

0.08

4.0

1,000

1,000

100

0.64

23.0

25% converted to heptanals

b)

Loading Rh(%)

(rax10 )

80% conversion to heptanals


90

POLYMERIC REAGENTS A N D CATALYSTS

Initial


1.0

0.5

29 on

PROPYLENE

days stream

& •H

O

0.3 0.2 0.1

8 days

MIXED OLEFINS

0

QJ 1.0

0.5 HEXENES

6 days

o L 0

50

100

D i s t a n c e Down Bed, P e r c en t Figure 3. Depletion of Rh with time as a function of the distance down the bed. Reproduced with permission from reference 14. 1977 Elsevier Science Publishers, B.U.

CHo-CH1 I PPH

2


5.

GARROU

Stability


Transition

Metal

91

Catalysts

r e a c t i o n s . G r e g o r i o and co-workers (17) f i r s t r e p o r t e d the slow decomposition of PPhj to PPh (n-Pr) during rhodium catalyzed pronene h y d r o f o r m y l a t i o n . l a t e r s t u d i e s by Tanaka and co-workers (18) i n Japan, A b a t j o g l o u and co-workers at Union Carbide (19,20) and our work with Allcock and Lavin (3,21) confirmed such r e s u l t s and pointed towards o x i d a t i v e - a d d i t i o n of the phosphorus-carbon bond as a plausible reaction mechanism. Primary degradation products from R3P were shown to be RH and RCHO. Secondary products as shown i n Scheme 1 a r e d e r i v e d f r o m h y d r o g e n a t i o n of RCHO and subsequent h y d r o g e n o l y s i s or homologation of RCH 0H. **lp NMR s t u d i e s have observed R PH when the h y d r o f o r m y l a t i o n r e a c t i o n i s run i n the absence of o l e f i n substrate and R PR when the reaction i s run i n the presence of o l e f i n . Such t e r t i a r y phosphine d e g r a d a t i o n has now been observed f o r Co, Rh, Ru, Pd, N i , and Os t e r t i a r y phosphine complexes (23,24). In addition to hydroformylation, such degradation has been observed i n carboxylation, hydrogenation and dehydrogenation r e a c t i o n s (23,24). A r y l group s c r a m b l i n g c a t a l y z e d by group 8-10 t r a n s i t i o n metals have a l s o been r e c e n t l y d e s c r i b e d (19.21) w i t h species such as 6 implicated as intermediates. It thus appears c l e a r that g i v e n the proper c i r c u m s t a n c e s , t e r t i a r y phosphines may become s i g n i f i c a n t l y involved i n the reaction c h e m i s t r y of the compounds of which they are a p a r t . With such chemistry i n mind, i t was thus not surprising that examination of the hydroformylation reaction catalyzed by by Co (C0)g supported on 2 or 7 revealed both loss of Co and loss of phosphorus (3.21). A dec 1ine i n c a t a l y t i c a c t i v i t y w i t h use was d e t e c t e d f o r r e a c t i o n s c a t a l y z e d by e i t h e r s p e c i e s . Polymers 2 and 7 i n the absence of c o b a l t both r e v e a l e d e x c e l l e n t s t a b i l i t y at 190°C ( h y d r o f o r m y l a t i o n temperatures). T h i s i s i l l u s t r a t e d by the TGA curves shown i n F i g u r e 4. Curve A shows an onset of decomposition for phosphinated polyphosphazene of 400°C, s l i g h t l y better than that of phosphinated p o l y s t y r e n e (curve B, 20% c r o s s l i n k e d ; curve C, 2% c r o s s l i n k e d ) . Loss of phosphorus was observed over a p e r i o d of 45 hours f o r a c a t a l y s t d e r i v e d from 2 (2% DVB c r o s s l i n k e d ) . The data d e p i c t e d i n F i g u r e 5 r e v e a l benzene, toluene, b e n z y l a l c o h o l , diphenylphosphine and triphenyl phosphine as cleavage products. I f one r e c a l l s the previously discussed homogeneous results i t should be clear that the PPh3 i s derived from a phosphido intermediate such as


2

2

2

f

2

2

8 •

Cleavage, as depicted, would give a PPhn cobalt complex. Such PPI13 would only b u i l d up i n solution to a small degree since i t would compete w i t h the r e s i n f o r bonding to the Co and would i t s e l f be susceptible to the phosphorus-carbon bond cleavage reaction. In a s i m i l a r fashion elemental analysis of recovered 7/Co (CO)g r e v e a l e d a 7% drop i n carbon and phosphorus, w h i l e the n i t r o g e n content remained constant. Such data i n d i c a t e that a l o s s of the pendant phosphine groups i s not occuring by a random chain scisson or d e p o l y m e r i z a t i o n . PPh and Ph PH can a l s o be be d e t e c t e d i n the reaction solution after catalysts derived from 7 have been f i l t e r e d 2

3

2




92

P R 3

Co (C0) , 2

g

» H /C0 2

P R o H + RH + R C H o O H i 2

— — > R C H 2 - »

3

RCH CH 0H 2

2

CO/Ho Scheme 1

R(L)

X

- Mr

M(L) R X

OPH

#

OPH

OPH

PPH

9

7



5.

GARROU

0

Stability


100

200

300

Transition

400

Metal

500

93

Catalysts

600

T«mp«ratur«

700

800

(°C)

Figure 4. TGA curves indicating s t a b i l i t y of Polymer 2 and 7 at 190 C i n the absence of cobalt. Curve A shows the onset of decomposition f o r phosphinated polyphosphazene of 400 C, s l i g h t l y better than that of phosphinated polystyrene (Curve B, 20$ cross-linked; Curve C» 2% crosslinked .


94


PhH

A |-{o)-PPh /Co (CO)


2

3

2

+

PhCH 0H

+

2

B

PhCH

3

+

Pl^PH

+

Ph P 3

C

>

8

.

i 2

1 '

2 U 6 8 10

20

30

40

50

hours

F i g u r e 5. Data r e v e a l i n g benzene, t o l u e n e , benzylalcohol, diphenylphosphine, and triphenylphosphine as c l e a v a g e p r o d u c t s .


5.

GARROU

Stability


Transition

Metal

Catalysts

95

i n agreement with results obtained for catalysts derived from 2. In a s i m i l a r f a s h i o n c l o s e a n a l y s i s of the h y d r o f o r m y l a t i o n catalysts derived from Co (CO)g and 9 (mw 161.000X22) revealed an 18% loss of phosphorous from the polymer and substantial lowering of the molecular weight.

off,

2


Formation of Metal C r y s t a l l i t e s Collman and co-workers (25) f i r s t r e p o r t e d the use of 2 to support t r a n s i t i o n metal species i n 1972 when they prepared complexes of Rh, Co, and Ir. They found that complexes of 2 when exposed to oxygen or 1 atmosphere of decomposed to metal p a r t i c l e s and that such catalysts were active for typical "heterogeneous" reactions such as the hydrogenation of arenes. They compared the a c t i v i t y of Rh$(C0)i6 on 2 to that of 5% Rh on A l o . Wilkinson s catalyst, (PPhn) RhC1, i s a very useful hydrogenation c a t a l y s t and has found great u t i l i t y i n the hands of synthetic organic chemists. I t i s not p o i s o n e d by s u l f u r f u n c t i o n a l i t y , i t i s selective for mono- and di-substituted o l e f i n s , i t does not c a t a l y z e h y d r o g e n o l y s i s and such c a t a l y s t s c o n t a i n i n g c h i r a l phosphine l i g a n d s permit asymmetric i n d u c t i o n . The major drawback to i t s use i s that i t has to be removed from the r e a c t i o n product by chromatography. T h i s i s time consuming, c o s t l y and wasteful of rhodium. V a r i o u s groups have sought through the years to i m m o b i l i z e Wilkinson-like rhodium catalysts on phosphinated resins to overcome such problems. Strukul and co-workers (26) have studied rhodium (I) supported on phosphinated p o l y s t y r e n e (2 or 20%) d i v i n y l b e n z e n e coploymers. C a t a l y s t s at v a r i o u s phosphorous and rhodium contents were examined for the hydrogenation of cyclohexene. They observed an induction period and catalyst a c t i v i t y dependent on the P/Rh r a t i o , the c a t a l y s t s becoming l e s s a c t i v e at h i g h e r P/Rh. A f t e r a few r e c y c l e s c a t a l y s t s having low P/Rh r a t i o s d i s p l a y e d a c t i v i t y i n s e n s i t i v e to the presence of the l i g a n d , i n c o n t r a s t to f r e s h catalyst or homogeneous analogues. In s i m i l a r fashion a c e t o n i t r i l e poisoned f r e s h c a t a l y s t but had no e f f e c t on r e c y c l e d c a t a l y s t . These r e s u l t s were i n t e r p r e t e d i n terms of c a t a l y s i s by rhodium metal• In a r e l a t e d study Guyot and coworkers (27) examined s i m i l a r catalysts varying the crosslinking by 2, 25, and 60% d i v i n y l benzene i n c o r p o r a t i o n . A f t e r rhodium f i x a t i o n the catalysts we tested for hydrogenation of 1-hexene i n e t h a n o l . The a c t i v i t y i n c r e a s e d w i t h d e c r e a s i n g rhodium content. Upon treatment w i t h H the c a t a l y s t turned g r e y - b l a c k . The c a t a l y s t s were a l s o examined f o r benzene hydrogenation a c t i v i t y . The a c t i v i t i e s were always low f o r f r e s h c a t a l y s t and i n c r e a s e d a f t e r hydrogen treatment or w i t h age. T h i s happened more q u i c k l y f o r the macroporous r e s i n s then f o r the g e l resins. If the H treatment was carried out at 100 °C the reduction 2

3

f

3

2

2



96

POLYMERIC REAGENTS AND CATALYSTS

to metal was a l s o f a c i l e f o r the g e l r e s i n s . There i s a l s o a r e l a t i o n s h i p to the Rh/P rat io, the a c t i v i t y f o r benzene hydrogenation developing more q u i c k l y f o r higher Rh/P ratios. S i m i l a r r e s u l t s were obtained f o r s i m i l a r Rh and I r c a t a l y s t s supported on 1 (27.27). Gates and co-workers (28) a l l owed Rh^(co)]^^ to r e a c t w i t h 2 (1.9-5.4% c r o s s l i n k e d ) at 25°C and produced c a t a l y s t s a c t i v e f o r cyclohexene hydrogenation. When the Rh^(CO)^ was allowed to react at 50 °C with the functionalized polymer, the r e s u l t i n g product was g r e y - b l a c k and TEM ( t r a n s m i s s i o n e l e c t r o n microscopy) revealed 254OA aggregated metal p a r t i c l e s on the catalyst surface. When oxygen was allowed to contact the catalysts the rhodium aggregation occurred more quickly. Only with polymers having unusually high P/Rh r a t i o s , i.e. 40/1, was t h i s phenomenon slowed down. S i m i l a r r e s u l t s were obtained for IrA(Co)^2 * analogous polymer supports (29). Although i t could be shown by IR that I r , c a r b o n y l c l u s t e r s were the predominant s p e c i e s present i n the f u n c t i o n i n g hydrogenation catalysts, t h e i r concentration did not correlate with reaction rates. It was thus q u e s t i o n e d whether c a t a b l y t i c a c t i v i t y was due to undetectably small amounts of metal aggregates. P i t t m a n and coworkers (30) s t u d i e d the r e a c t i o n of butadiene w i t h c a r b o x y l i e a c i d s to g i v e octadieny1 e s t e r s c a t a l y z e d by Pd loaded phosphine functionalized styrene-divinylbenzene r e s i n beads (1% DVB). Although the resins were recycled they noted a s i g n i f i c a n t decrease i n a c t i v i t y . The Pd content dropped, for example, from 1.74 to 1.29% after 2 recycles and to 1.05% a f t e r 5 recycles. In addition to Pd loss they noted Pd metal deposited i n the r e s i n matrix. Higher P/Pd r a t i o s appeared to i n h i b i t metal agglomeration. 0

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Polymeric Reagents and Catalysts - ACS Publications - American

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